Fruit neighborhoods and interactions between birds and plants in Puerto Rico

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Title: Fruit neighborhoods and interactions between birds and plants in Puerto Rico
Author: Saracco, James Frederick
Advisors: T. R. Simons, Committee Member
T. R. Wentworth, Committee Member
K. H. Pollock, Committee Member
M. J. Groom, Committee Co-Chair
J. A. Collazo, Committee Co-Chair
Abstract: Many species of plants that produce fleshy fruits depend on birds for seed dispersal, and many of the birds that disperse seeds rely on fruits for a substantial proportion of their diets. From an ecological and evolutionary standpoint, it is important to understand why frugivorous birds feed in the particular plants they do. Intrinsic plant characteristics (e.g., crop size) influence the foraging patterns of birds; however, these factors cannot be fully understood outside the context of the communities within which birds and plants find themselves. Here I report on spatial patterns of bird use of fruiting plants in central Puerto Rico and its relationship to plant distributions and fruit abundance. From Feb.-Jun. 1998 I quantified frugivorous bird visitation to fruiting trees of Schefflera morototoni in a secondary forest-shade coffee plantation mosaic. Visitation rate was positively related to crop size and negatively related to conspecific fruit abundance within 30 m of focal plants. The presence or abundance of heterospecific bird-consumed fruits had mixed relationships with visitation: four species were positively related to visitation; one was negatively related. The fruit neighborhood explained most variation in visitation to focal trees, suggesting that such variables might help explain the high variability seen in other fruiting plant frugivore systems. In order to evaluate spatial dependency in tree distributions, fruiting, and frugivory over a range of tree species and spatial scales, I mapped bird-consumed fruiting plants on a 4.05 ha study grid in secondary wet forest and monitored fruiting and frugivory from Aug.-Nov. 1999. I focused analyses on four tree species: Dendropanax arboreus, Guarea guidonia, Miconia serrulata, and Schefflera morototoni. All of these were intraspecifically aggregated at scales < 80 m, but differed markedly in degree of crowding experienced by individuals. G. guidonia was the most, and M. serrulata the least crowded. Distributions of visited trees and autocorrelation in the number of frugivory observations at trees suggested that individuals of some species (e.g., S. morototoni) facilitated visitation to one another at small spatial scales (< ˜30 m). Frugivory was positively correlated with fruit abundance on trees for all species; spatial variation in fruit abundance appeared to have reduced, and in some cases outweighed (e.g., M. serrulata), facilitative benefits of visited neighbors. Evidence of facilitation was especially weak for G. guidonia and may have reflected its particularly high density in the study area. Consideration of interspecific tree distributions showed S. morototoni to be aggregated with M. serrulata at scales > 69 m, and G. guidonia to be aggregated with D. arboreus at scales < 5 m. All other pairs were randomly or regularly distributed with respect to one another. Spatial patterns of fruiting and frugivory of M. serrulata appeared linked to the distribution of fruiting S. morototoni. Spatial patterns of frugivory also overlapped for other species and generally suggested facilitation at larger scales. Interspecific interactions were probably strongest at larger scales because of shifts in the relative abundances of conspecifics and heterospecifics at those scales. These findings highlight the potentially large size of plant neighborhoods with respect to use by avian frugivores and the dependence of neighborhood effects on local plant densities and crop sizes. From the perspective of birds, spatial patterns of frugivory suggested birds closely tracked fruit abundance. I observed few agonistic interactions between birds and found little evidence of their negatively influencing one another's use of fruiting trees. Similarity in spatial patterns of frugivory between bird species, and positive cross-correlation in frugivory of different species at patch boundaries, suggested birds may have assisted one another (via calling) in locating new foraging patches. This could explain mixed-species flock formation.
Date: 2002-08-19
Degree: PhD
Discipline: Zoology

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